powerpc: Create a powerpc64 config feature

The config symbol TARGET_ps3 is used in the build files where
special handling for the 64 bit PowerPC processors is needed.
Introduce a new config feature powerpc64, and replace the use
of TARGET_ps3 with powerpc64.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>

SVN-Revision: 17306

target/Config.in

@@ -100,6 +100,9 @@ config powerpc
 	select BIG_ENDIAN
 	bool
 
+config powerpc64
+	bool
+
 config sh3
 	bool